Over time you may find your Gmail inbox filling up. Messages with image and document attachments can be a part of that problem since they are larger than ordinary messages. It's a good idea to remove those messages, if they are not needed anymore, and free up a lot of space in your Gmail account. Of course, you can keep them but if you run out of space in your Gmail account, you have to buy extra storage space from Google.
Finding those large messages is easy. In the Gmail's search bar you can run this query to find emails larger than 5 megabytes: